Aircraft Communication System
K. Seetharamaraju (134103062)
Basic Radio Principles
• Communication System involves voice transmission and reception between aircrafts or aircrafts and ground.
• For obvious reasons pilots can not use sound as a medium during the flight to interact with other pilots !
• So we need a specialized system which can play an efficient role of transmitting important data( voice, signals) and
simultaneously maintaining a good quality transmission.
• It can be said that safe aircraft operation is dependent to a large extent upon satisfactory performance of
• Radios are employed for fulfilling the purpose
• The First World War brought about an urgent need for communications. Voice
communications from ground-to-air and from aircraft to aircraft were established
• In the 1930s, the first all radio-controlled blind-landing was accomplished. By the end of
the decade, use of high frequency radio waves was explored and included the advent of
• World War II injected urgency into the development of aircraft radio communication and
navigation. Communication radios, despite their size, were essential on board aircraft.
Very high frequencies were developed for communication and navigational purposes.
• From the basic theory we know that the energy that illuminates the light is transmitted
by an alternating electromagnetic field in the core of transformer.
• Thus it can be said that it is a kind of wireless control of one circuit(secondary) by
• A basic radio communication works on similar transformer principle which involves
transmission and reception of electromagnetic(radio) waves through space.
• Alternating current passing through a conductor creates electromagnetic field around the conductor.
• Energy is alternately stored in these field and returned to the conductor for typical transformer
• As the frequency of current alternation increases, lesser energy stored in the field returns to the
conductor and instead radiated into space in form of EM waves.
• Now for Radios, this phenomenon is exploited for transmission of signals and the conductor employed
to radiate in this manner is called transmitting antenna.
• Now if these radiated EM waves passes through a conductor, they transmit some energy into
the conductor by setting conductor’s electron into motion.
• As and when EM field varies, pattern of electron flow changes which in turn varies the
• Hence by causing a variation into the EM field via radiating antenna, we can bring about
similar variation in the receiving antenna.
• This is the basic theory of radio transmission
• The Radio Frequencies are being used in various field, amongst them include :
• FM Radio Broadcast
Land Mobile Stations (Emergency, Business, and Military)
AIR TRAFFIC CONTROL
AIR NAVIGATION SYSTEMS (e.g.VOR/DME,NDB)
• The use of the various Radio Frequencies all over the World is allocated by ITU
(International Telecommunications Union.
• The radio frequency portion of the electromagnetic spectrum extends from 30 kHz
(kilohertz) to 30,000 MHz (Megahertz).
• For the sake of convenience, this spectrum is divided into frequency bands
• Transmitters may be considered as a generator which changes electrical power into
• It basically performs these functions:
i) Generate RF ( radio frequency) signal
ii) Amplify the RF signal
iii) Provide a means of placing intelligence on the signal
Components of Transmitter
o Oscillator circuit to generate RF signal
o Amplifier circuit to increase the output of the oscillator to the required power level
o Modulator to add voice (audio) intelligence to the RF signal. Modulation can be
further done in two ways ,(a) Amplitude Modulation (AM) and, (b) Frequency
• Just opposite to transmitters, receivers
i) Select the desired radio frequency signals
ii) Amplifies the generated small a.c. voltage
iii) Convert the intelligence contained on these signals into a usable form
with the help of demodulator. Demodulator which is used for AM signals
is called detector, while the one which is used for FM waves is called
• An antenna is a special type of electrical circuit designed to radiate and receive
• As mentioned earlier, a transmitting antenna is a conductor which radiates EM waves
when a radio frequency is passed through it, while receiving antenna intercepts the EM
waves that are present in the air.
• They come in various size and shapes depending upon the frequency to be transmitted.
• Generally for aircraft application, same antenna is used for both transmission and
reception of signals.
Antenna as transition device
Printed Circuit Antenna
Types of antenna
Antenna on Aircraft
• Type and size of antenna varies with the different types of aircraft.
• The location of antenna depends on the design of aircraft.
VHF and HF
• VHF and HF communication systems use transceivers.
• A transceivers is a self contained transmitter and receiver.
• The transmitter and receiver both operate at same frequency, and the
microphone button determines when there is an output from the transmitter.
• In the absence of transmission the receiver is sensitive to incoming signals.
• This combination of transmitters and receivers saves a lot of space and
weight and are hence largely used.
Very High Frequency
• Very High Frequency is a term used to describe the 30MHz. to 300MHz.
portion of the radio spectrum.
• This range of frequencies will provide short-range LOS (line of site)
• The range for VHF communications depends on equipment used, antenna
height, and terrain(typically 2 to 20 miles).
• The frequency range is 3-30 MHz.
• This is used for long range communications because of its longer
• This is the basic band for long-range communications, mainly because its
transmissions are reflected from the ionosphere.
• HF transmitters have higher power outputs than VHF transmitters.
• A Tuner is used to match the impedance of the transceiver to the antenna.
• Use and installation of the HF communication system is same as that of the
VHF communication system.
• The advantage of using HF communication system over VHF
communication system is that it can transmit information over long distance
as the output power of the HF transmitter is higher than that of the VHF